Refractory insulator for electron discharge devices



Patented Oct. 6, 1931 UNITED STATES PATENT oFFlcej mum: H. names, or ZBLOOMFIELD, NEW JERSEY. assmuon 'ro WESTINGHOUSE LAM? COMPANY, aooarona'rron or PENNSYLVANIA REFRACTORY INSULATOR FOR ELECTRON DISCHARGE DEVICES '7 I This invention relates to insulating material for use in vacuum electric devicesand more particularly to an insulating support for the heater element of an indirectly heated thermionically active cathode.

The'type of electron discharge device with which the invention is particularly concerned employs a cathode heated by radiation and conduction from an electrically insulated heating element energized by the passage of alternating current therethrough. The heating element of such a cathode may consist of a tungsten filament which is supported within an aperture in a cylindricalor other shaped insulator of porcelain, isolantite, or other refractory material about which is positioned a metal cylinder coated with a thermionically active material such as the oxides of the alkaline earth metals. This invention is an improvement in the invention described in copending application Serial No. 233,543, filed Nov. 16, 1927 ,by John W. Marden and Frank H. Driggs and assigned to the same assignee as the present invention. p I a In said copending application is described and claimed the method of preparation and an insulating support for use in indirectly heated cathodes, comprised of the oxide or oxides of the type of thoriumloxide (ThO The invention in said copending application was directed to a means of overcoming certain deleterious effects developing in electron discharge devices employing indirectly heated cathodes through the use therein of insulating support members reactive with respect to the heater element or which contained therein a relativelylarge residual gas content which is evolved and is reactive with respect to the heater element therein when'the cathode is heated to elevated temperatures.

Whereas the utilization of the oxides of the type of thorium oxide (ThO ZrO raTe earth oxides, etc.) have resulted in a pronounced improvement in the manufacture of, life and maintenance of saidelectron discharge devices. the improved state of the art and more rigid manufacturing and operating conditions require that the insulating support member of the composite cathode be sub Am lication med September 15, 1928. Serial No; 806,291.

stantially of smaller cross sectional area and stronger in transverse strength than heretofore obtainable.

It is one of the objects of the presentinvention to materially increase thelife, strength and maintenance of the insulating support member of the composite indirectly heated cathode made from substantially inert oxides of the type thorium oxide.

It is also one of the objects of the present invention to increase the operating-efliciency of the indirectly heated cathode and the electron discharge devices produced therefrom.

Another object of this invention is to simplify and improve themanufactu-re and assembly of saidindirectly heated cathodes and electron discharge devices produced therefrom.

In'accordance with my invention I add to the relatively inert refractory oxide having a relatively high melting point a proportion of a suitable refractory material having a lower melting point so as to substantially cffeet a lowering of the .sinteringor fritting I.

isolantite, talc magnesium, silicate, andthe like, or any of the various compounds ormixtures of compounds of silica'with magnesia and alumina of the type formulas 9X MgOY SiO or X A1 0 Y SiO or mixtures of the two wherein X. and Y represent varying percentages of the, two constituents. m The amounts of these constituents added to the refractory inert oxide varies somewhat, depending upon the specific refractory oxide employed, the specific additive employed, and with respect somewhat to the diameter of the insulator support desired. The total amount of the additive employed will vary from 110% (by weight). i v v I have determinedthat whereassatisfactory insulating supports may be made by the processset'forth in said copen'ding application Serial No. 233,543, the limitations of available furnace sintering temperatures impose a manufacturing lower limit upon the cross sectional diameter permissible to obtain through the use of such refractory materials alone.

The highest commercially available furnace temperatures which may be employed in the firing of the insulator supports are obtained in the tungsten wound or molybdenum wound hydrogen furnaces, and lies between 1500 C. and 1600 C. Higher temperatures may be obtained but are not practical from a commercial production standpoint.

Inasmuch as the maximum temperatures attained in the heater element of the composite cathode employing the insulating support member does not usually exceed 1400 C. to 1600 C. itis not essential for the success of the application of the thoria (ThO or zirconia (ZrO type insulators to rigidly retain this high melting point characteristic. It is, however, essential to the usefulness of these materialsto retain the relative chemical inertness of the oxides.

In addition, it must be appreciated that owing to the high melting points of these oxides it is difficult to fire these refractories at the available furnace temperatures 1500 to 1600 C. and develop therein the maximum density, coherence and transverse strength.

In the manufacture of electron discharge devices employing the indirectly heated cathode, it has been found that variations in density and coherence of the material comprising the insulator support materially effects the heat conductivity of the support member, the electrical resistivity of the same and the strength of the support. Variations in the heat conductivity effects the time interval of heating up the electron emitting cathode, the necessary operating temperature of the heater element and it is highly essential that both factors be kept at a minimum. The strength of the insulator is of prime importance from a manufacturing standpoint and should be kept as high as possible so that the cross sectional diameter of the support and hence the heat conducting path be limited. The electrical resistivity of the material must be high to avoid the transference by conduction through the insulator electrical currents of any of the lower frequencies to the therinionically active metallic sleeve surrounding the insulator, as this introduces a low frequency hum in radio circuits utilizing the device.

As a specific embodiment of my invention I prefer to employ thorium oxide as the refractory oxide and 5% (by weight) isolantit e which is an artificially prepared and highly purified magnesium silicate. I have employed from 1% to 10% of talc and various mixtures of magnesium and aluminum silicates but have found that the so-called iso'lantite material which may be obtained in a highly purified condition and which may be finely ground to effect more ready distribution in the refractory oxide, is most satisfactory for my purpose.

After thoroughly ball milling (dry) the mixture of 95% ThO and 5% magnesium silicate (isolantite) is admixed with a binder substantially in a manner as described and claimed in said copending application Serial No. 233,543 and the mass extruded in the desired shape and sizes. The extruded material after air drying to harden may then be cut into suitable lengths, placed in molybdenum boats and are then fired in tungsten or molybdenum wound hydrogen furnaces to temperatures ranging between 1500 C. to 1600 C. for from 2 to 3 hours.

The resulting product obtained differs from the pure thoria insulators heretofore prepared in having a glazed appearance and in having a relatively low porosity. The strength and resistance to transverse stresses is materially increased. Due to the effect of the increased density and decreased porosity the electrical resistance has been increased to such an extent over pure thoria insulators fired similarly that whereas with said pure thoria insulators using a bias of 9 volts between the heater element (filament) and the nickel sleeve of the cathode, a current leakage of approximately 0.8 to 0.9 milliamperes may be detected, through the use of an additive bonding agent such as described; this current leakage under the same test conditions is reduced to 0.01 to 0.02 milliamperes. This materially improves the operating efficiency of such composite cathodes.

Having broadly outlined the scope of my invention and specifically described one method of utilizing the same in the production of improved insulator supports for cathodes of the indirectly heated type it is apparent that many variations may be made from the specific embodiment without departing from the scope of the invention as encompassed within the following claims.

hat is claimed is:

1. An electron-emitting cathode composed of a hollow metal body having a thermionically active surface, a heating element contained therein and an insulator disposed between said heating element and said hollow metal body, said insulator being comprised of substantially pure thorium oxide and approximately 5% magnesium silicate.

2. As an article of manufacture, vitrified dense formed refractory ware comprised of thorium oxide (ThO and approximately 5% magnesium silicate.

In testimony whereof, I have hereunto subscribed my name this 14th day of September, 1928.

FRANK H. DRIGGS. 

